Dual frequency band communication antenna assembly having an inverted F radiating element

ABSTRACT

A dual frequency band antenna has a first section with a dielectric material separating parallel first and second conductive layers. An electrical shunt extends between the first and second electrically conductive layers. A second section includes an inverted F element electrically connected to the second electrically conductive layer. A transmission medium carries signals between the antenna assembly and a communication circuit and has a first electrical conductor connected to the first electrically conductive layer and a second electrical conductor connected to the inverted F element.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to antennas for two-way communication,such as radio equipment in vehicles and mobile telephones, and moreparticularly to planar antennas for such applications.

2. Description of the Related Art

Computers, data terminals, and other electronic equipment in vehicles,such as police cars, employ radios to exchange data and otherinformation with base stations. For example, cellular telephone networksand WIFI Internet connections are commonly used for communication withsuch mobile equipment. The radio system that links the mobile electronicequipment to the telephone network or the Internet has an antenna on theexterior of the vehicle to send and receive the radio frequency signals.Cellular telephones transmit in the 824 to 845 MHz frequency band andreceive signals in the 870 to 896 MHz frequency band. PCS telephonesoperate in the 1850 to 1990 MHz. frequency band. The WIFI protocolenables communication over different frequency bands, for example the2.4 GHz ISM band and the 5.0 GHz U-NII band. An antenna that is tuned tooperate with one of these frequency bands is not optimum forcommunication in another frequency band.

A typical communication antenna for a motor vehicle is attached to theexterior surface of the roof or trunk and comprises a short section ofrigid wire extending vertically. Separate antennas typically arerequired in order to communicate on multiple frequency bands. Eventhough such antennas are relatively short, protruding about one footfrom the surface of the vehicle, they are subject to accidentalbreakage, such as in automatic car washes, and acts of vandalism. Theseantennas are often considered to be unsightly and a detraction from theaesthetic appearance of the vehicle.

U.S. Pat. No. 5,041,838 describes a low profile, flat disk-shapedantenna for bidirectional communication, such as cellular telephones.This antenna is attached to a horizontal exterior surface of the motorvehicle, such as the roof. A coaxial cable extends through a hole inthat surface, coupling the external antenna to the transceiver insidethe motor vehicle. This antenna is tuned to a single frequency band.

U.S. Pat. No. 6,087,990 discloses a low profile, flat disk-shapedantenna assembly that combines two antennas into a single package. Oneantenna is tuned for bidirectional communication equipment, such ascellular telephones, while the other antenna in designed for anothertype of radio frequency equipment, such as a global positioningsatellite receiver. Separate coaxial cables for each type of equipmentconnect to this dual antenna assembly.

U.S. Pat. No. 6,850,191 describes an antenna assembly has a pair of diskshaped antennas, each tuned to a different frequency band, therebyenabling the same assembly to be used with two different ofcommunication apparatus. One antenna disk lies on top of the other inelectrical contact. A single coaxial cable carries the signals for bothantennas with one conductor of the cable attached to one antenna and theother conductor is attached to the other antenna.

SUMMARY OF THE INVENTION

An antenna assembly comprises a first antenna section and a secondantenna section for transmitting and/or receiving signals in twodifferent frequency bands.

The first antenna section includes a first electrically conductive layerextending in a first plane, a second electrically conductive layerextending in a second plane that is spaced from and parallel to thefirst plane, and a dielectric material between the first and secondelectrically conductive layers. An electrical shunt is connected to thefirst and second electrically conductive layers.

The second antenna section comprises an inverted F element that iselectrically connected to the second electrically conductive layer. Inone embodiment, the inverted F element includes a rod of electricallyconductive material which has a L-shape with a first leg and a secondleg that is longer than the first leg. An end of the first leg iselectrically attached to the second electrically conductive layer andthe second leg is parallel to the second electrically conductive layer.

A transmission medium for carrying signals between the antenna assemblyand a communication circuit has first and second electrical conductors.The first electrical conductor is connected to the first electricallyconductive layer and the second electrical conductor connected to theinverted F element. For example, the second electrical conductor isconnected to the second leg, thereby forming the short third leg of theinverted F element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plane view of the top of dual frequency band antennaassembly according to the present invention; and

FIG. 2 is a cross sectional view along line 2-2 in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The phrase “directly connected to” as used herein means that theassociated components are electrically connected together without anyintervening element, other than a connector, through which electricitymust flow to be conducted from one directly connected component to theother component. The term “directly connecting” means that therespective component connects two other components without anyintervening element, other than a connector, through which electricitymust flow.

With reference to FIGS. 1 and 2, a dual frequency band antenna assembly10 is mounted on a surface of an object 12, such as a roof of a motorvehicle. The antenna assembly 10 comprises a first antenna section 16for communication at a first frequency and a second antenna section 18for communication at a second frequency.

The first antenna section 16 is formed with a circular disk-shapedsubstrate 20 of a dielectric material, such as PMI foam or a PTFEcomposite. The diameter of the substrate 20 is less than one-half thewavelength of the radio frequency signals which the first antennasection is to transmit and receive. Limiting the diameter in this matterprevents high order modes from being excited. For frequencies bandscommonly used for WIFI transmission, the substrate 20 is 1.5 inches indiameter and 0.375 inches thick, for example.

The bottom and top flat major surfaces 23 and 25 on opposite sides ofthe substrate 20 are in parallel planes and have geometric centers thatlie on a common axis 26. First and second conductive layers 21 and 22are respectively mounted on the bottom and top major surfaces 23 and 25.For example, the conductive layer may be formed by brass or copperplates bonded to those major surfaces. Although the first and secondconductive layers are separated by a body of dielectric material, thesubstrate 20 may be eliminated by separating the two conductive layers21 and 22 by air, which also is a dielectric material. The firstconductive layer 21 covers the entirety of the substrate's bottom majorsurface 23. The second conductive layer 22 is substantially centered onthe top major surface 25 and extends over only a portion of thatsurface. As shown in FIG. 1, the second conductive layer 22 has atear-drop shape with an outwardly projecting tip 24. Specifically, thesecond conductive layer 22 has a circular major portion 27 that iscentered on the top major surface 25 of the substrate 20 and from whichthe tip 24 projects. Thus the second conductive layer 22 issubstantially centered on the common axis 26.

For example, if the first antenna section 16 is to operate in the 2.4GHz ISM frequency band, the substrate 20 and the first conductive layer21 may be approximately 1.5 inches in diameter. The circular majorportion 27 of the second conductive layer 22 may be 0.68 inches indiameter with the tip 24 extending approximately 0.43 inches from thecenter point of the major portion, which center point is on axis 26.Therefore, the flat surface area of the first conductive layer 21 ismore that four times the flat surface area of the second conductivelayer 22.

A conductive tuning post 29 extends through the first conductive layer21, the dielectric substrate 20, and the tip 24 of the second conductivelayer 22, thereby electrically directly connecting the first and secondconductive layers. A brass or copper tuning post may be used. The tuningpost 29 can be a hollow rivet with heads at both ends that are solderedto the respective conductive layer. Alternatively, the tuning post 29may be first inserted through the substrate 20 and then the first andsecond conductive layers 21 and 22 deposited on the major surfaces ofthe substrate in electrical contact with the tuning post. One skilled inthe art of antenna design will appreciate that the precise number andlocations of the tuning posts are a function of the radio frequencies tobe received and/or transmitted by the antenna.

An aperture 28 extends through the first antenna section 16 along thecommon axis 26 and thus through the centers the circular disk-shapedsubstrate 20 and the first and second conductive layers 21 and 22.

The second antenna section 18 is mounted on the second conductive layer22 on the top major surface 25 of the substrate 20. The second antennasection 18 has an inverted F element 40 that includes a conductive rod41 bent in an L-shape, thereby having a relatively short first leg 42and a longer second leg 44. The end of the first leg 42 is affixed inelectrical contact to the second conductive layer 22 offset from thecommon axis 26 at the center of that layer. The second leg 44 extendsparallel to the plane of the second conductive layer 22 and intersectsthe common axis 26.

For a second antenna section 18 that operates in the 5.0 GHz U-NII band,the shorter first leg 42 may be 0.128 inches in length and attached tothe second conductive layer 22 at a point 0.083 inches from the commonaxis 26. The longer second leg 44 may have a length of 0.350 inches. Theaxis of the second leg 44 can be oriented 45 degrees from a line thatintersects the common axis 26 and the tuning post 29. The conductive rod41 may be formed of copper with a diameter of 0.032 inches.

A conventional coaxial cable 30 forms a transmission medium thatconnects the antenna assembly 10 to a communication circuit, such as aradio transceiver. The shield conductor of the coaxial cable 30 isdirectly connected electrically by a connector 32 to the firstconductive layer 21 on the bottom major surface 23 of the first antennasection 16. A center conductor 34 and an insulator layer 36 of thecoaxial cable 30 extend into the aperture 28 in the first antennasection 16. The center conductor 34 projects through and outwardly fromthe second conductive layer 22 terminating at a remote end 38. Thecenter conductor 34 is spaced from the second conductive layer 22 so asto be electrically isolated therefrom. As shown in FIG. 2, the remoteend 38 of the center conductor 34 is attached to the underside of thesecond leg 44 of the L-shaped conductive rod 41, thereby forming theshort third leg 46 of the inverted F element 40.

The antenna assembly 10 can operate at two cellular telephonefrequencies or two frequencies of N-WIFI. For N-WIFI, the first antennasection 16 may be tuned to operate at 2.4 GHz ISM band, while the secondantenna section 18 tuned for the 5.0 GHz U-NII band. At thosefrequencies, each antenna section 16 and 18 is in essence electricallyinvisible to the other. Thus, for the first antenna section 16, thefirst conductive layer 21 acts as the ground plane and the secondconductive layer 22 serves as the radiating element. The signal for thefirst antenna section 16, that is carried by the center conductor 34 ofthe coaxial cable 30, travels directly through the conductive rod 41into the second conductive layer 22 exciting that layer to radiate thesignal.

The inverted F element 40 acts as the radiating element of that secondantenna section 18 and the second conductive layer 22 functions as theground plane. In other words, at the higher signal frequency (e.g., 5.0GHz), the structure of the first antenna section 16 is in essenceinvisible to the second antenna section 18 and the electrical couplingprovided by the tuning post 29 makes the second conductive layer 22appear as though it was connected directly to the shield conductor ofthe coaxial cable 30. Therefore, in the present antenna assembly 10, thesecond conductive layer 22 functions as the radiating element of thefirst antenna section 16 and as the ground plane for the second antennasection 18.

The foregoing description was primarily directed to a preferredembodiment of the invention. Although some attention was given tovarious alternatives within the scope of the invention, it isanticipated that one skilled in the art will likely realize additionalalternatives that are now apparent from disclosure of embodiments of theinvention. Accordingly, the scope of the invention should be determinedfrom the following claims and not limited by the above disclosure.

What is claimed is:
 1. An antenna assembly comprising: a first antennasection including a first electrically conductive layer that has a firstsurface area in a first plane, a second electrically conductive layerthat has a second surface area in a second plane that is spaced from thefirst plane, a dielectric material between the first and secondelectrically conductive layers, and an electrical shunt connected to thefirst and second electrically conductive layers, wherein the firstsurface area is at least two times the second surface area; a secondantenna section comprising an inverted F element attached to the secondelectrically conductive layer; and a first electrical conductor and asecond electrical conductor for carrying signals between the antennaassembly and a communication circuit, the first electrical conductorconnected to the first electrically conductive layer and the secondelectrical conductor connected to the inverted F element.
 2. The antennaassembly as recited in claim 1 wherein the first plane is parallel tothe second plane.
 3. The antenna assembly as recited in claim 1 whereinthe dielectric material comprises a body having a first surface on whichthe first electrically conductive layer is mounted and a second surfaceon which the second electrically conductive layer is mounted.
 4. Theantenna assembly as recited in claim 3 wherein the body is disk shaped.5. The antenna assembly as recited in claim 1 wherein the first surfacearea is more than four times the second surface area.
 6. The antennaassembly as recited in claim 1 wherein the second electricallyconductive layer has a tear-drop shape with an outwardly projecting tip,and the electrical shunt is located adjacent the tip.
 7. The antennaassembly as recited in claim 1 wherein the inverted F element comprisesa rod of electrically conductive material having an L-shape with a firstleg and a second leg that is parallel to the second electricallyconductive layer, wherein an end of the first leg is attached to thesecond electrically conductive layer, and wherein the second leg islonger than the first leg and has the second electrical conductorattached thereto.
 8. The antenna assembly as recited in claim 1 whereinthe first electrical conductor is attached to a center portion of thefirst electrically conductive layer, and the second electrical conductorextends through an aperture in a center region of the secondelectrically conductive layer.
 9. An antenna assembly comprising: afirst antenna section having a substrate of dielectric material withfirst and second major surfaces, a first electrically conductive layeron the first major surface, a second electrically conductive layer onthe second major surface and having a tear-drop shape with an outwardlyprojecting tip, and at least one electrical shunt located proximate tothe tip of the second electrically conductive layer and attached to thefirst and second electrically conductive layers; a second antennasection comprising an inverted F element having a rod of electricallyconductive material that has a L-shape with a first leg and a second legthat is longer than the first leg, wherein an end of the first leg isdirectly connected to the second electrically conductive layer and thesecond leg is parallel to the second electrically conductive layer; anda transmission medium for carrying signals between the antenna assemblyand a communication circuit, and comprising a first electrical conductorconnected to the first electrically conductive layer and a secondelectrical conductor connected to the second leg.
 10. The antennaassembly recited in claim 9 wherein the substrate is disk shaped. 11.The antenna assembly as recited in claim 9 wherein the substrate is acircular disk.
 12. The antenna assembly as recited in claim 9 wherein anaxis extends through a center of the first electrically conductive layerand through the second electrically conductive layer, the first leg isconnected to the second electrically conductive layer at a point that isoffset from the axis, and the second leg intersects the axis at alocation where the second electrical conductor is connected to thesecond leg.
 13. The antenna assembly as recited in claim 12 wherein thefirst electrical conductor is connected to the first electricallyconductive layer adjacent the axis.
 14. The antenna assembly as recitedin claim 9 wherein the first and second electrically conductive layersare substantially centered on a common axis.
 15. The antenna assembly asrecited in claim 9 wherein the first major surface and the second majorsurface are parallel.
 16. The antenna assembly as recited in claim 9wherein the first electrically conductive layer has a first surface areaagainst the first major surface, and the second electrically conductivelayer has a second surface area against the second major surface,wherein the first surface area is at least twice the second surfacearea.
 17. The antenna assembly as recited in claim 16 wherein the firstsurface area is more than four times the second surface area.
 18. Anantenna assembly comprising: a first antenna section including a firstelectrically conductive layer in a first plane, a second electricallyconductive layer in a second plane that is spaced from the first plane,a dielectric material between the first and second electricallyconductive layers, and an electrical shunt connected to the first andsecond electrically conductive layers, wherein the second electricallyconductive layer has a tear-drop shape with an outwardly projecting tip,and the electrical shunt is located adjacent the tip; a second antennasection comprising an inverted F element attached to the secondelectrically conductive layer; and a first electrical conductor and asecond electrical conductor for carrying signals between the antennaassembly and a communication circuit, the first electrical conductorconnected to the first electrically conductive layer and the secondelectrical conductor connected to the inverted F element.
 19. Theantenna assembly as recited in claim 18 wherein the first electricallyconductive layer has a first surface area in the first plane, and thesecond electrically conductive layer has a second surface area in thesecond plane, wherein the first surface area is at least twice thesecond surface area.